[Qemu-devel] [PATCH 2/2] net: netmap: avoid mmap() when ports use the same shared memory area

[Vincenzo Maffione]

With this patch, nm_open() does not mmap() the netmap device. This
operation is performed separately only if the memory area of the
port just opened was not known before.
A global list of netmap clients is kept to check when matches
occur.

Signed-off-by: Vincenzo Maffione <address@hidden>
---
 net/netmap.c | 38 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 37 insertions(+), 1 deletion(-)

diff --git a/net/netmap.c b/net/netmap.c
index 27295ab..6a4c01c 100644
--- a/net/netmap.c
+++ b/net/netmap.c
@@ -49,8 +49,12 @@ typedef struct NetmapState {
     bool                write_poll;
     struct iovec        iov[IOV_MAX];
     int                 vnet_hdr_len;  /* Current virtio-net header length. */
+    QTAILQ_ENTRY(NetmapState) next;
 } NetmapState;
 
+static QTAILQ_HEAD(, NetmapState) netmap_clients =
+                   QTAILQ_HEAD_INITIALIZER(netmap_clients);
+
 #ifndef __FreeBSD__
 #define pkt_copy bcopy
 #else
@@ -78,6 +82,23 @@ pkt_copy(const void *_src, void *_dst, int l)
 #endif /* __FreeBSD__ */
 
 /*
+ * find nm_desc parent with same allocator
+ */
+static struct nm_desc *
+netmap_find_memory(struct nm_desc *nmd)
+{
+    NetmapState *s;
+
+    QTAILQ_FOREACH(s, &netmap_clients, next) {
+        if (nmd->req.nr_arg2 == s->nmd->req.nr_arg2) {
+            return s->nmd;
+        }
+    }
+
+    return NULL;
+}
+
+/*
  * Open a netmap device. We assume there is only one queue
  * (which is the case for the VALE bridge).
  */
@@ -86,10 +107,11 @@ static struct nm_desc *netmap_open(const 
NetdevNetmapOptions *nm_opts,
 {
     struct nm_desc *nmd;
     struct nmreq req;
+    int ret;
 
     memset(&req, 0, sizeof(req));
 
-    nmd = nm_open(nm_opts->ifname, &req, NETMAP_NO_TX_POLL,
+    nmd = nm_open(nm_opts->ifname, &req, NETMAP_NO_TX_POLL | NM_OPEN_NO_MMAP,
                   NULL);
     if (nmd == NULL) {
         error_setg_errno(errp, errno, "Failed to nm_open() %s",
@@ -97,6 +119,17 @@ static struct nm_desc *netmap_open(const 
NetdevNetmapOptions *nm_opts,
         return NULL;
     }
 
+    /* Check if we already have a nm_desc that uses the same memory as the one
+     * just opened, so that nm_mmap() can skip mmap() and inherit from parent.
+     */
+    ret = nm_mmap(nmd, netmap_find_memory(nmd));
+    if (ret) {
+        error_setg_errno(errp, errno, "Failed to nm_mmap() %s",
+                         nm_opts->ifname);
+        nm_close(nmd);
+        return NULL;
+    }
+
     return nmd;
 }
 
@@ -321,6 +354,8 @@ static void netmap_cleanup(NetClientState *nc)
     netmap_poll(nc, false);
     nm_close(s->nmd);
     s->nmd = NULL;
+
+    QTAILQ_REMOVE(&netmap_clients, s, next);
 }
 
 /* Offloading manipulation support callbacks. */
@@ -423,6 +458,7 @@ int net_init_netmap(const NetClientOptions *opts,
     s->rx = NETMAP_RXRING(nmd->nifp, 0);
     s->vnet_hdr_len = 0;
     pstrcpy(s->ifname, sizeof(s->ifname), netmap_opts->ifname);
+    QTAILQ_INSERT_TAIL(&netmap_clients, s, next);
     netmap_read_poll(s, true); /* Initially only poll for reads. */
 
     return 0;
-- 
2.7.0